1. Field of the Invention
The present invention relates to semiconductor wafer cleaning and drying and, more particularly, to apparatuses and techniques for the usage of acoustic waves in conjunction with a fluid meniscus to more efficiently clean wafer surfaces and reduce contamination during wafer processing.
2. Description of the Related Art
In the semiconductor chip fabrication process, it is well-known that there is a need to clean and dry a wafer where a fabrication operation has been performed that leaves unwanted residues on the surfaces of wafers. Examples of such a fabrication operation include plasma etching (e.g., via etch or trench etch for copper dual damascene applications) and chemical mechanical polishing (CMP). In CMP, a wafer is placed in a holder which pushes a wafer surface against a rolling conveyor belt. This conveyor belt uses a slurry which consists of chemicals and abrasive materials to cause the polishing. Unfortunately, this process tends to leave an accumulation of slurry particles and residues at the wafer surface. If left on the wafer, the unwanted residual material and particles may cause, among other things, defects such as scratches on the wafer surface and inappropriate interactions between metallization features. In some cases, such defects may cause devices on the wafer to become inoperable. In order to avoid the undue costs of discarding wafers having inoperable devices, it is therefore necessary to clean the wafer adequately yet efficiently after fabrication operations that leave unwanted residues.
The use of acoustic energy is a highly advanced, non-contact, cleaning technology for removing small-particles from substrates such as semiconductor wafers in various states of fabrication, flat panel displays, micro-electro-mechanical systems (MEMS), micro-opto-electro-mechanical systems (MOEMS), and the like. The cleaning process typically involves the propagation of acoustic energy through a liquid medium to remove particles from, and clean, a surface of a substrate. The megasonic energy is typically propagated in a frequency range of between about 600 kHz (0.6 Megahertz (MHz)) to about 1.5 MHz, inclusive. The typical liquid medium that can be utilized is deionized water or any one or more of several substrate cleaning chemicals and combinations thereof such as a dilute ammonium hydroxide/hydrogen peroxide solution in DI water. The propagation of acoustic energy through a liquid medium achieves non-contact substrate cleaning chiefly through the formation and collapse of bubbles from dissolved gases in the liquid medium, herein referred to as cavitation, microstreaming, and chemical reaction enhancement when chemicals are used as the liquid medium through improved mass transport, optimizing the zeta potential to favor particle entrainment in the liquid medium and inhibiting re-deposition, or providing activation energy to facilitate the chemical reactions.
FIG. 1A is a diagram of a typical batch substrate cleaning system 10. FIG. 1B is a top view of the batch substrate cleaning system 10. A tank 11 is filled with a cleaning solution 16 such as deionized water or other substrate cleaning chemicals. A substrate carrier 12, typically a cassette of substrates, holds a batch of substrates 14 to be cleaned. One or more transducers 18A, 18B, 18C generate the emitted acoustic energy 15 that is propagated through the cleaning solution 16. The relative location and distance between the substrates 14 and the transducers 18A, 18B and 18C are typically approximately constant from one batch of substrates 14 to another through use of locating fixtures 19A, 19B that contact and locate the carrier 12.
The emitted energy 15, with or without appropriate chemistry to control particle re-adhesion, achieves substrate cleaning through cavitation, acoustic streaming, and enhanced mass transport if cleaning chemicals are used. A batch substrate cleaning process typically requires lengthy processing times, and also can consume excessive volumes of cleaning chemicals 16. Additionally, consistency and substrate-to-substrate control are difficult to achieve.
FIG. 1C is a prior art, schematic 30 of an RF supply to supply one or more of the transducers 18A, 18B, 18C. An adjustable voltage controlled oscillator (VCO) 32 outputs a signal 33, at a selected frequency, to an RF generator 34. The RF generator 34 amplifies the signal 33 to produce a signal 35 with an increased power. The signal 35 is output to the transducer 18B. A power sensor 36 monitors the signal 35. The transducer 18B outputs emitted energy 15.
Unfortunately, the typical megasonic system has the problem of slow chemical exchange and a large effective reactor chamber volume. This can lead to contaminants being left in a megasonic reaction chamber to be redeposited on the wafer. Consequently, this can lead to inefficient cleaning and lowered wafer processing yields. Further, hot spots or cold spots in the batch cleaning system can be generated by constructive or destructive interference of the acoustic wave due to reflections from the substrates and tank walls. These hot or cold spots can either damage sensitive structures present on the substrate, or cause inefficient or non-uniform cleaning. Therefore, there is a need for a method and an apparatus that avoids the prior art by enabling quick and efficient cleaning of a semiconductor wafer, but at the same time reducing the redeposition of contaminants on the wafer following a cleaning operation while using low amounts of cleaning fluid as well as providing a uniform power density delivery to the substrate without hot or cold spots. Such deposits of contamination as often occurs today reduce the yield of acceptable wafers and increase the cost of manufacturing semiconductor wafers.